Material for structural components of an electrowinning cell for production of metal

ABSTRACT

A material suitable for use for structural components in a cell for the electrolytic reduction of alumina to aluminium metal defined either by: 
         the formula (A′ 1−u A″ u ) x (B′ 1−v B″ v ) y (C′ 1−w C″ w ) z O 4 , in which A′ and A″ are divalent elements from the group Co, Ni, or Zn, B′ and B″ are trivalent elements from the group Al, Cr, Mn, or Fe, and C′ and C″ are the tetravalent elements Ti or Sn. O is the element oxygen. 0≦u&lt;1, 0≦v&lt;1, 0≦w&lt;1 1≦x≦2, 0≦y≦2 and 0≦z≦1, x+y+z=3 and 2x+3y+4z=8,  or    the formula A′ 1−s A″ s TiO 3 , in which A′ and A″ are divalent elements from the group Co, Ni, or Zn. O is the element oxygen. 0≦s&lt;1  or    the formula A′ 1−t A″ t O, in which A′ and A″ are divalent elements from the group Co, Ni, or Zn. O is the element oxygen. 0≦t&lt;1.

FIELD OF INVENTION

The present invention relates to a material that can be used forstructural components in a cell for the electrolysis of aluminadissolved in a fluoride containing molten salt bath by the use ofessentially inert electrodes.

BACKGROUND ART

Conventionally, aluminium is produced by the electrolysis of aluminadissolved in a cryolite based molten salt bath by the more than hundredyears old Hall-Heroult process. In this process carbon electrodes areused, where the carbon anode is taking part in the cell reactionresulting in the simultaneous production of CO₂. The gross consumptionof the anode is up to 550 kg/tonne of aluminium produced, causingemissions of greenhouse gases like fluorocarbon compounds in addition toCO₂. For both cost and environmental reasons the replacement of carbonanodes with an effectively inert material would be highly advantageous.The electrolysis cell would then produce oxygen and aluminium.

An earlier, not yet disclosed Norwegian patent application No. 2001-0927describes the development and design of a novel electrowinning cell foraluminium production. The novel cell is based on vertical electrodetechnology and a two chamber electrolysis cell for separation ofproduced metal and evolved oxygen gas. The cell concept requires thatcertain structural elements are made of materials that must fulfil theirfunctional requirements at elevated temperatures in an environment of amolten fluoride-based electrolyte. In some regions of the cell anadditional requirement is that the materials must fulfil theirfunctional requirements in contact with liquid aluminium, while in otherregions the materials must fulfil their functional requirements incontact with pure oxygen gas at a pressure of about one bar.

OBJECT OF THE INVENTION

The object of the present invention is to identify a material that isstable at an oxygen partial pressure of 1 bar at temperatures aboveabout 680° C. and has a sufficiently low solubility in the electrolyteto be used as a material for structural cell components in oxidizingregions of aluminium electrowinning cell based on substantially inertelectrodes.

SUMMARY OF THE INVENTION

The invention is the conclusion of an extensive search for materialscapable of fulfilling the requirements for a material for structuralcell components in oxidizing regions of an aluminium electrowinning cellbased on substantially inert electrodes. The stability requirements ofsuch a material are similar to those of an inert anode in saidelectrowinning cell. In the not yet published Norwegian PatentApplication No. 2001-0928 the choice of possible element oxides for aninert anode is narrowed to: TiO₂, Cr₂O₃, Fe₂O₃, Mn₂O₃, CoO, NiO, CuO,ZnO, Al₂O₃, Ga₂O₃, ZrO₂, SnO₂ and HfO₂. The main requirements for amaterial intended for use in structural cell components are stability at1 bar oxygen pressure at temperatures above 680° C. and a low solubilityin the molten electrolyte. The electrical properties are less important,but its electrical conductivity should be for less that the electricalconductivity of the electrodes and the electrolyte. The material shouldeither itself fulfil the requirements, or it should upon contact withthe molten electrolyte react to form a surface layer of an aluminatethat fulfils the said requirements. Based on solubility considerations,CuO, Ga₂O₃, ZrO₂ and HfO₂ are eliminated from the list of possibleelement oxides, and we are left with: TiO₂, Cr₂O₃, Fe₂O₃, Mn₂O₃, CoO,NiO, ZnO, Al₂O₃, and SnO₂.

The evaluation leads to a family of materials that can be messed by theformula(A′_(1−u)A″_(u))_(x)(B′_(1−v)B″_(v))_(y)(C″_(1−w)C″_(w))_(x)O_(t)in which A′ and A″ are elements from the group Co, Ni or Zn, B′ and B″are elements from the group Al, Cr, Mb, or Fe, and C′ and C″ are theelements Ti or Sn. O is the element oxygen. 0≦n<1, 0≦v<1, 0≦w<1 1≦x≦2,0≦z≦1, and t is a number that renders the composition charge neutral.

Within this group of oxides, materials most commonly crystallize in thespinel ilmenite or rock salt structures. In materials of the inventionthat possess the spinel structure, x+y+z=3, 2'y+4z=8, and t=4. Inmaterials of the present invention that possess the ilmenite structure,x+y+z=2, 2x+3y+4z=6 and t=3. In materials of the present invention thatpossess the rock salt structure, x=1, y=z=0, and t=1.

DETAILED DESCRIPTION OF THE INVENTION

A material suitable as an essentially inert material for structuralcomponents in the oxidizing regions of a cell for the electrolyticproduction of aluminium alumina dissolved in an essentially fluoridebased electrolyte where cryolite is an important ingredient, must beresistant to oxidation and dissolution in the electrolyte. A selectionof the element oxides which a material for structural components canconsist of, was performed based on the following criteria:

-   -   not a gas or having a high vapour pressure at process        temperature    -   not converted by cryolite or AlF₃ in the cryolitic mixture, i.e.        a large positive value of ΔG° for the reaction between the        element oxide and AlF₃ to form the element fluoride and        aluminium oxide (reaction 1).        MO_(x)+2x/3AlF₃=MF_(2x)+2x/6Al₂O₃  (1)    -   not converted by alumina, i.e. not a negative value of ΔG° for        the reaction between the element oxide aluminium oxide and        sodium fluoride to form a sodium element oxide and aluminium        fluoride (reaction 2)        MO_(x)+6yNaF+yAl₂O₃═Na_(6y)MO_(x+3y)+2yAlF₃  (2)

Of elements with the normal valence 2, the only possible elements arethus Co, Ni, Cu and Zn. Of elements with valence 3 one is left with onlythe elements Cr, Mn, Fe, Ga and Al. Of elements with valence 4 one isleft with only the elements Ti, Zr, Hf, Ge and Sn. Cu, Ga, Zr, Hf and Gemay be eliminated from the list based on solubility considerations, andwe are left with the following list of elements: Co, Ni, Zn, Al, Cr, Mn,Fe, Ti and Sn. The possible materials for structural cell components inan aluminium electrowinning cell based on substantially inert electrodesare thus limited to the oxides of the listed elements, or combinationsof these oxides in mixed oxide compounds.

The materials within this group can be expressed by the formula:(A′_(1−u)A″_(u))_(x)(B′_(1−v)B″_(v))_(y)(C″_(1−w)C′_(w))_(z)O_(t)  (a)in which A′ and A″ are divalent elements from the group Co, Ni, or Zn,B′ and B″ are trivalent elements from the group Al Cr, or Fe, and C′ andC″ are the tetravalent elements Ti or Sn. O is the element oxygen.0≦u<1, 0≦v<1, 0≦w<1 1≦x≦2, 0≦y≦2, 0≦z≦1, and t is a number that rendersthe composition charge neutral.

Under favourable conditions the divalent oxides NiO, CoO and ZnO allreact with alumina to form an essentially insoluble surface aluminatelayer (reaction 3).AO(s)+Al₂O₃(diss)=AAl₂O₄(s)  (3)where A=Co, Ni, Zn. Therefore, CoO, MO and ZnO and solid solutions ofthese form one group of possible materials for structural cellcomponents. These compositions am messed by formula (a) with x=1, y=z=0,and t=1. This is further illustrated in Examples 1 and 2.

Compounds of di- and trivalent element oxides will in this case be ofthe spinel structure. Spinels like NiFe₂O₄, CoFe₂O₄, NiCr₂O₄ and CoCr₂O₄have been suggested and extensively tested as candidates for inertanodes. In these materials, Al from the molten electrolyte has beenobserved to exchange with the trivalent cation to form essentiallyinsoluble, insulating solid solutions of the type Ni(B′_(1−v)Al_(v))₂O₄,where 0<v<1, B′=Fe, Cr, Mn. This is further illustrated in Examples 3,4, and 6. These material are thus possible materials for structural cellcomponents. The pure aluminates NiAl₂O₄, CoAl₂O₄ and ZnAl₂O₄ are alsopossible materials for structural cell components.

One compound of di- and tetravalent element oxides, Zn₂SnO₄, forms aspinel oxide. This material may in principle be used for structural cellcomponents.

Other stable spinel compositions that are possible materials forstructural components of an aluminum electrowinning cell are achieved bysubstituting a divalent/trivalent spinel with a tetravalent oxide, whilesimultaneously adjusting the contents of the divalent and trivalentoxides in order to maintain the site and charge balance requirements ofthe spinel structure. This embodiment of the present invention isexemplified in Example 5.

Spinel type materials thus form another subset of materials forstructural components of aluminium electrowinning cells. Thesecompositions are expressed by formula (a), with x+y+z=3, 2x+3y+4z=8, andt=4.

NiTiO₃, CoTiO₃ and solid solutions of these crystallize with theilmenite structure. A cell material with the ilmenite structure may alsobe obtained by substitution of a trivalent element from the list ofpossible elements for equimolar amounts of divalent and tetravalentelements. These compositions are expressed by formula (a) with x+y+z=32,+y+4z=6, and t=3.

The invention shall in the following be further described by figures andexamples where:

FIG. 1: Shows a photograph of a sample of a material for structuralcomponents in an electrolysis cell before and after the stability testof Example 3.

FIG. 2: Shows a backscatter SEM photograph of the reaction zone of aNi_(1.1)Cr₂O₄ material after 50 hours of exposure to molten fluorideelectrolyte under anodic polarization.

FIG. 3: Shows a backscatter SEM photograph of a NiFeCrO₄ sample after 50hours of exposure to molten fluoride electrolyte under anodicpolarization.

FIG. 4: Shows a backscatter SEM photograph of a sample ofNi_(1.5+x)FeTi_(0.5−x)O₄ after the stability test of Example 5.

FIG. 5: Shows a backscatter SEM photograph of a Ni_(1.0)Fe₂O₄ sampleafter 30 hours of exposure to molten fluoride electrolyte under anodicpolarization.

EXAMPLE 1 Test of the Stability of a NiO Sample Anodically Polarized ina Molten Fluoride Electrolyte

A cermet with 75 wt % NiO and 25 wt % Ni was prepared using INCO Nipowder type 210, and NiO from Merck, Darmstadt. The material wassintered in argon atmosphere at 1400° C. for 30 min.

The sample was exposed to a molten fluoride bath under anodicpolarization in order to ensure a partial pressure of 1 bar oxygen onthe sample surface. The electrolyte was contained in an alumina cruciblewith inner diameter 80 mm and height 150 mm. An outer alumina containerwith height 200 mm was used for safety, and the cell was covered with alid made from high alumina cement. In the bottom of the crucible a 5 mmthick TiB₂ disc was placed, which made the liquid aluminium cathode stayhorizontal. The electrical connection to the cathode was provided by aTiB₂ rod supported by an alumina tube to avoid oxidation. A platinumwire provided electrical connection to the TiB₂ cathode rod. A Ni wireprovided for the electrical connection to the anode. The Ni wire and theanode above the electrolyte bath was masked with an alumina tube andalumina cement to prevent oxidation.

340 g Al, (99.9% pure), from Hydro Aluminium was placed on the TiB₂ discat the bottom of the alumina crucible.

The electrolyte was made by adding to the alumina crucible a mixture of:

-   -   532 g Na₃AlF₆ (Greenland cryolite)    -   105 g AlF₃ (from Norzink, with about 10% Al₂O₃)    -   35 g Al₂O₃ (annealed at 1200° C. for some hours)    -   21 g CaF₂ (Fluka p.a.)

The sample of the material for structural cell components was suspendedabove the electrolyte during heating of the cell. The temperature wasmaintained at 970° C. during the whole experiment. The sample of thematerial for structural cell components was lowered into the moltenelectrolyte and polarized anodically with a current density of 750mA/cm² based on the bottom end cross sectional area of the sample. Thereal current density was somewhat lower because the side surfaces of theanode were also dipped into in the electrolyte.

The experiment lasted for 8 hours. XRD (X-ray diffraction) analysis ofthe anode after the experiment showed that the Ni metal was oxidized toNiO and the anode material was covered by an dense, protective,insulating layer of NiAl₂O₄.

EXAMPLE 2 Test of the Stability of a ZnO Sample Anodically Polarized ina Molten Fluoride Electrolyte.

ZnO was doped with 0.5 mol % AlO_(1.5). Two Pt wires were pressed intothe material in the longitudinal axis of the ZnO anode and acted aselectrical conductors. The material was sintered at 1300° C. for 1 hour.

The stability test was performed in the same manner as described inExample 1. The amounts of electrolyte and aluminium were the same. Thetemperature was 970° C. The current density was set to 1000 mA/cm² basedon the bottom end cross sectional area of the sample The electrolysisexperiment lasted for 24 hours. XRD (X-ray diffraction) analysis of thesample after the electrolysis experiment showed that ZnO had beenconverted ZnAl₂O₄ during electrolysis.

EXAMPLE 3 Test of the Stability of a Ni_(1+x)Cr₂O₄ Sample AnodicallyPolarized in a Molten Fluoride Electrolyte

The starting powder was prepared by a soft chemistry route. Theappropriate amounts of Ni(NO₃)₂, and Cr(NO₃)₃ were complexed with citricacid in dilute nitric acid. After evaporation of excess water, themixture was pyrolysed and calcined at 900° C. for 10 hours. The samplewas cold isostatically pressed at 200 MPa, then sintered at 1440° C. for3 hours. The material was found by XRD to possess the spinel structure.

The stability test was performed in the same manner as described inExample 1, but a platinum wire provided electrical connection to thesample. The platinum wire to the sample was protected by a 5 mm aluminatube. When the electrolysis started the anode was dipped approximately 1cm into the electrolyte. A photograph of the sample before and afterelectrolysis is shown in FIG. 1.

The electrolyte, temperature and current density were the same asdescribed in Example 2.

The stability test lasted for 50 hours. After the experiment the samplewas cut, polished and examined in SEM (Scanning Electron Microscope). Areaction zone could be seen between the Ni_(1.1)Cr₂O₄—material and theelectrolyte. FIG. 2 shows the backscatter SEM photograph of the reactionzone. On the photograph one can see a reaction zone that has propagatedalong the grain boundaries of the Ni_(1.1)Cr₂O₄ material. The whiteparticles are NiO.

In the table below the relative EDS analysis results are reported. Ni,Cr, Al, and O were the only elements detected. The aluminium present inthe interior of the grains might be due to the preparation of the samplefor analysis. Relative comparison between the elements Ni, Cr and Al:Atom % in the centre Atom % in the reaction zone Element: of the grainsin FIG. 2: in grain boundaries in FIG. 2: Ni 33 47 Cr 66 8 Al 1 45

The SEM analysis shows that the reaction product consisted of a materialwhere the chromium atoms were partly exchanged with aluminium atoms asdescribed by the formula NiCr_(2−x)Al_(x)O₄ where x varies from 0 to 2.The reaction product forms an insulating coating.

EXAMPLE 4 Test of the Stability of a NiFeCrO₄ Sample AnodicallyPolarized in a Molten Fluoride Electrolyte

The starting powder was prepared by a soft chemistry route. Theappropriate amounts of Ni(NO₃)₂, Fe(NO₃)₃ and Cr(NO₃)₃ were complexedwith citric acid in dilute nitric acid. After evaporation of excesswater, the mixture was pyrolysed and calcined at 900° C. for 10 hours.The sample was cold isostatically pressed at 200 MPa, then sintered at1600° C. for 3 hours. The material was found by XRD to possess thespiniel structure.

The stability test was performed in the same manner as described inExample 3. The amounts of electrolyte and aluminium were the same. Thecurrent density was set to 1000 mA/cm² based on the cross sectional areaof the rectangular sample. The experiment lasted for 50 hours.Examination of the sample after exposure to molten fluorides underanodic polarization showed a several micron thick reaction layer whereCr in the material was partly exchanged with Al atoms. A backscatter SEMphotograph of the reaction layer is shown in FIG. 3. Light grey areasconsist of original NiFecrO₄ material. Medium grey area contains almostno Cr atoms and a much lower content of Fe.

EDS analysis of the medium grey reaction layer shown in FIG. 3 comparedto original NiFeCrO₄ material and the inner of the anode light grey areaalso shown in FIG. 3 are summarized in table below. The only elementsdetected were Ni, Cr, Fe, Al and O. Comparison of the relative amountsof Cr, Fe, Ni and Al: Atom % in the original Atom % in the reactionNiFeCrO₄ material. layer after the test. Element: Light grey area inFIG. 3. Medium grey area in FIG. 3. Cr 33.3 0 Fe 33.3 16 Ni 33.3 35 Al 049

The conclusion of the stability test is that the NiFeCrO₄ materialreacts with alumina in the electrolyte and forms a dense, essentiallyinsoluble, insulating layer of NiFe_(1−x)Al_(1+x)O₄.

EXAMPLE 5 Test of the Stability of a Ni_(1.5+x)FeTi_(0.5−x)O₄ SampleAnodically Polarized in a Molten Fluoride Electrolyte

The starting powder was prepared by a soft chemistry route. Theappropriate amounts of Ni(NO₃)₂, Fe(NO₃)₃ and TiO₅H₁₄C₁₀ (titanylacetylacetonate) were complexed with citric acid in dilute nitric acid.After evaporation of excess water, the mixture was pyrolysed andcalcined at 900° C. for 10 hours. The sample was cold isostaticallypressed at 200 MPa, then sintered at 1500° C. for 3 hours. The materialwas found by XRD to possess the spinel structure.

The stability test was performed in the same manner as described inExample 3. The amounts of electrolyte and aluminum were the same. Thecurrent density was set to 1000 mA/cm² based on the cross sectional areaof the rectangular sample. The experiment lasted for 30 hours. After theexperiment the sample was cut, polished and examined in SEM. Thebackscatter photo in FIG. 4 shows the end of the sample facing thecathode. In this experiment no reaction layer was detected on theNi_(1.5+x)FeTi_(0.5−x)O₄ anode after 30 hours.

EXAMPLE 6 Test of the Stability of a Ni_(1.01)Fe₂O₄ Sample AnodicallyPolarized in a Molten Fluoride Electrolyte

The starting powder was prepared by a soft chemistry route. Theappropriate amounts of Ni(NO₃)₂, and Fe(NO₃)₃ were complexed with citricacid in dilute nitric acid. After evaporation of excess water, themixture was pyrolysed and calcined at 900° C. for 10 hours. The samplewas cold isostatically pressed at 200 MPa, then sintered at 1450° C. for3 hours. The material was found by XRD to possess the spinel structure.

The stability test was performed in the same manner as described inExample 3. The amounts of electrolyte and aluminium were the same. Thecurrent density was set to 1000 mA/cm² based on the cross sectional areaof the rectangular anode. The experiment was stopped after 30 hours.After the experiment the sample was cut, polished and examined in SEM.FIG. 5 shows a backscatter photograph of the sample at the end facingthe cathode. An approximately 10 micron thick reaction layer is seen.

A line scan EDS analysis was done to examine whether the layer was areaction layer or electrolyte adhering to the surface. The line scanindicated a thin layer of bath components, and then a reaction layer ofapproximately 10 micron thickness. In the interior of the anode and inthe reaction layer only oxygen was detected in addition to Ni, Fe andAl. The results are reported in the table below: Comparison of therelative amounts of Ni, Fe and Al: Atom % of element in Atom % ofelement in the interior of the anode the reaction layer as shown in FIG.5 and analysed shown in FIG. 5 and analysed Element: with line scan EDS:with line scan EDS: Ni 33 30 Fe 67 30 Al 0 40

In the 10 micron thick reaction layer the iron atoms were partlyexchanged with aluminium atoms to form an essentially insoluble,insulating layer of NiFe_(2−x)Al_(x)O₄.

1-4. (canceled)
 5. An electrolysis cell for electrolytic reduction ofalumina to aluminum, which comprises structural components comprising amaterial which is thermodynamically stable and essentially electricallynon-conductive and further insoluble in a cryolite based melt with highalumina activities, the material being expressed by the general formula:A_(x)B_(y)C_(z)O₄, where A is a cationic element selected from the groupconsisting of Ni, Zn and Co, B is a cationic element selected from thegroup consisting of Fe, Cr, Al and Mn, C is a cationic element selectedfrom the group consisting of Al, Ti and Sn, O is the element oxygen,0<x≦1,0<y≦2,0≦z≦1, and the material contains at least two cations, one thereof beingAl.
 6. The electrolysis cell in accordance with claim 5, wherein thecationic element A is divalent Ni or Zn, the cationic element B is Al,x=1, y=2 and z=0.
 7. The electrolysis cell in accordance with claim 5,wherein the cationic element A is Ni, the cationic element B is Cr andthe cationic element C is Al where x is
 1. 8. The electrolysis cell inaccordance with claim 5, wherein the cationic element A is Ni, thecationic element B is Fe and the cationic element C is Al where x is 1.